From what I've been reading, there are strong proponents of SIPs and ICFs. It seems if properly built either should last a long time. For an equivalent size structure, which insulates best? Which costs less?

Best insulated is up for debate as ICFs depend on thermal mass which may or may not work for you depending on your climate.
SIPs cost less and in our area and Steel SIPs are less now than conventional block and truss.
While the ICF walls are strong, the typical strapped truss roofs are still suspect in hurricane conditions

And thermal-mass makes the most difference with wide swings in outdoor temperatures. I got into ICFs as much for the other benefits as much as the energy efficiency. Strength, lack of sound transmission, does not support mold growth and can support heavy floor systems and roofs. ICFs are the best overall system, when something better comes along I will do that.

So are you saying that ICFs are more for hot-dry and or hot-humid, but not for the northeast where we get hot humid in the summer and cold-wet in the winter? The house I'm thinking about will be a weekend house till I retire- at least 15 years. I like the idea of a programable thermostat that keeps the house above freezing in the winter mon-fri afternoon, then warms up for fri night-sun afternoon. I'm thinking a house with a lot of thermal mass might not be good for this, as the mass needs to be warmed. But what I'm really interested in is which insulates best and which costs less for an equivalent size structure?

ICFs are a better choice, in every part of the country. SIPs and ICFs use the same type insulation (usually), EPS. With the same amount of insulation ICFs will be more energy efficient. But to determine cost, have your project bid with both. I would hope ICFs cost more, but better is usually more expensive.

wow what a bold unbiased statement, not true in the SE ....ICF homes are running about 12-15% more , while our steel SIPs are competitively priced against standard block/truss running about the same to 2% more and in the acase of commercial work about 5% less.

thermal mass is not effective in all parts of the country

ICFs are difficult to alter easily after built , while with sips construction one can add a window or door opening in a matter of hours after the house is built

It is of course a bias opinion. Not because it is my business. ICFs are my business, because they are better.

Chuckle. And How & Why is that? Chuckle...

As someone who has no vested interest in bricks, sticks, SIPs or ICFs, I think you need a structure that is; i) VERY tight and ii) very well insulated, in that order.   Very well insulated to me means a minimum of 6" of styrofoam for your part of the world.  You can get that with "lopsided" ICF or SIPS.  If you relax the insulation requirements a bit, you can get there with sticks although tight becomes a bit harder.     For a weekend retreat, you want fast thermal response and while thermal mass is not a benefit, is probably not a big negative, either.

You also need to consider cost, the knowledge/experience base of the local construction industry and other special requirements such as wind resistance (tornado/hurricane), fire resistance, ease of DIY (if that is a consideration), schedule, etc.

I will not be so bold as to suggest that there is a universal "best" solution.  You can get good (or bad) results with any of the systems. 

Make a basic design (or create an Owner's Brief and have a design made) and shop it around for budgeting and "value engineering" purposes.  Until you have a design, any pricing you get is at best an educated WAG. 

Bruce

panelcraft,
Hehe, do you disagree? if so why? hehe.

Just for grins, Why would ICF's be more energy efficient(given the same amount of foam)?

Because to the concrete thermal mass.

Ah, yes...The infamous Thermal Mass...If you study up a bit, you'll be disappointed...Sorry...

Fords and Chevys!

I have, I'm not.

Thermal mass works good in the right climate and in the right conditions.

Pros: My house has a 6'*8' solid (yes solid) masonry fireplace right in the middle. In the spring and fall, we just turn off the heat/ac and just let the house do the work. In the summer/winter it makes our heat/ac stay on longer when it runs and then have a longer off period.

Cons: Those nice programmable thermostats that adjust the temp when you are not there do not work well with thermal mass. All that thermal heat you built up will be lost while you are away, and you will have to re-raise the temp long before to get the house to even out in temp. In that time, the hvac will cycle on/off a lot more than usual due to the thermal mass being recharged.

Don't know if thermal mass has anything to do with that, we have a SIPs home that is capable of going long periods of time staying comfortable without conditioning heat or cold. In winter months it maintains a 30 degree temperature difference when 36 degrees outsde and no heat inside. I think the heat off of lighting, appliances and bodies is sufficient to keep it no lower than 66 inside

Thermal Mass has everything to do with that. Everything in your house (air, floor, drywall, ect.) has thermal mass. The house I am in isn't superinsulated, over a long period of time the temperature will continue to get much cooler, but the amount of thermal mass slows that by continuing to source heat into the house as the house cools relative to the thermal mass. For those that know about electricity (I'm an EE) it is like the time decay in an RLC circuit, where Resistance is similar to your wall's R value and Capacitance is similar to your thermal mass.

ICF's aren't exactly like this, as they have an R value between the inside of the house and the thermal mass, but typical thermal mass with direct inside contact works great.

Based on that statement every house relies on thermal mass

Exactly! If your house had no thermal mass, it would cool off instantly, no matter what the R value of the walls were as soon as the heat kicked off. However, since our houses have air, furniture, drywall, ect. they take some time to cool down when your heating source is removed. It is not a question of if your house has thermal mass or not, it is just how much. Some materials have more thermal mass than others. Some of the best commonly available materials are water, drywall, and masonry.

Incorporating a lot of these materials directly exposed to the inside will make your house take longer to heat up and cool down than an identical house without as much thermal mass. This isn't an opinion, its a fact. The mass will lose heat into your house if your house cools lower than the mass almost instantly, thus keeping your house at the same temperature as the mass.

However, ICF’s do not act as a thermal mass in the same way as described above. ICF's have their thermal mass in between two pieces of foam. Its mass is not directly exposed to your inside air, so it's effectiveness as a thermal mass is questionable. In order to source any heat, it would have to flow through a piece of foam several inches thick. Also, the thermal mass will approach the weighted average between the inside and outside temperatures using the interior and exterior foam R values as the weights.

What does that mean in a nutshell? On days when it is to warm during the day and to cold at night, ICF thermal mass would help quite a bit by storing some of the hot over to the cold cycle and vice versa. On days when it is to hot all day or to cold all night, the ICF thermal mass wouldn’t do very much at all due to its inability to directly source heat into the inside to lower on/off cycles of your heater. If it was 32 degrees outside, and then it warmed up to 52 degrees rather quickly, your house would still be leaking heat at the 32 degree rate until the thermal mass had heated up to the new weighted average.

So what does all that mean in a nutshell? Directly exposed masonry is best, but masonry in between foam is still better than nothing in most climates.